There are two noteworthy differences between the X-ray emission from the ICM and IGrM. Firstly, in the hot ICM of clusters of galaxies, the free-free bremsstrahlung continuum is the dominant radiation process [1]}. Conversely, for plasma temperatures around and below 1 keV, which are typical of galaxy groups, increasing contributions to the continuum level come also from (i) recombination radiation, caused by the capture of an electron by an ion, leading to a spectral shape characterized by sharp ionization edges, and (ii) the slow transition from the 2s to the 1s state, which is forbidden by angular momentum conservation but can happen as a very slow two-photon process giving rise to continuum emission [2]}.
This makes it more difficult to determine the equivalent width of a given spectral line (EW\(EW=\int {(I_\nu -I_\nu ^0)}{I_\nu ^0} \:{\rm d}(h \nu )\) , where \(I_\nu ^0\)  is the continuum intensity without the line), a quantity that serves as a main diagnostic of the abundance of the chemical element from which that line originates.
